One of the most essential substances found in nature is clay and its minerals. With a better understanding of clay structure, montmorillonite (MMT) and other clays are now recognized as having the potential for improved performance in a variety of materials and goods, including packaging, polymers, adsorbents, flame retardancy, food additives, drug delivery, catalysis, etc. Thus, in recent years, there has been a significant advancement in the use of MMT. This introductory chapter summarizes the structure of diverse clays, characteristics, and current applications in significant materials. The natural origins, chemical composition, physical and chemical characteristics, and functional applications of clays, mainly nanoclays are discussed. Particle and layered structure, molecular structure and cation exchange effect, barrier property, and water sorption are a few of the significant physical and chemical features of nanoclays. Nanocomposites made of nanoclays are organic–inorganic hybrid materials with at least one filler dimension of less than 100 nm. The features of nanocomposites have been established by literature to significantly improve because the filler platelets disperse within the polymer at the nanoscale due to the exceptional filler surface changes. The boost in the characteristics typically only calls for a small amount of filler, helping the composite materials maintain their original transparency and low density.